The trick is at first only use a staple every couple inches and be prepared to remove some later.
I start by putting a staple in the middle of each side. I will then work one side towards the back only popping in a few staples. Do not go crazy with the staples yet. I will then move to the other side and work towards the back stretching the material as I go. When you have wrinkles you need to work them out as you go. If you wait until the end you will have a tough time removing them. After I have the back wrinkle free then I move back to the middle and go towards the front. Do not be afraid to remove previous staples. When you have the cover form fitted then go over it with a bunch of staples